Storage systems are frequently categorized according to their availability, a term that refers to the ability of users and their applications to access the storage system. A storage system is considered unavailable if a user application cannot access the storage system, for example, to read or write information. Mission-critical applications, for example, financial transactions, medical care, nuclear power operations, require a high level of availability of their systems. Users of such high availability systems are generally intolerant of system unavailability and generally require an absolute degree of operational continuity during a specified period. For other types of applications, availability is certainly expected, but not critical; users of low availability systems possess a degree of tolerance for unavailability. However, low availability systems experiencing excessive downtime or performing poorly can fail the expectations of their users. Often, service level agreements specify the expected level of availability, thereby establishing metrics against which actual performance of the system may be measured.
Storage systems designed for high availability generally implement redundancy at various systems levels: for example, redundant storage processors, redundant power supplies, redundant link control cards, redundant arrays, and redundant Fibre Channel loops. In general, purchasers are able to specify the level of redundancy of a storage system only at the time of its purchase. These purchasers often have in mind at the time of purchase the highest level of availability that will be required of their storage systems, and therefore initially configure the redundancy of their storage systems to satisfy that anticipated level. Often, though, users of the storage systems do not require this high level of availability around-the-clock for every day of the week and every day of the year.
In addition, data centers are continuously growing larger, with their storage systems ever expanding in number and in storage capacity. Usually, the data centers configure their storage systems to operate at optimized input/output (I/O) performance and system response time. Often, though, little or no consideration is given to managing the overall power consumption of the storage system. Thus, the storage systems run continuously at their maximum power consumption. This continuous operation increases the total power dissipated and, consequently, the cost of ownership to the data centers.